1.0 Telecommunications Networks
In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipments (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS, i.e. Universal Mobile Telecommunications System) or “eNodeB” or “eNB” (LTE, i.e. Long Term Evolution,). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC)) which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
2.0 UMTS
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The Universal Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access (WCDMA) for user equipment units (UEs). In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate e.g. enhanced data rate and radio capacity.
3.0 Long Term Evolution (LTE)
The 3GPP has developed specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN). The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE). Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected to a core network (via Access Gateways, or AGWs) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (i.e. eNodeBs in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
3.1 LTE Overview
LTE uses OFDM in the downlink and DFT-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms., as shown by FIG. 2.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks (RB), where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
The notion of virtual resource blocks (VRB) and physical resource blocks (PRB) has been introduced in LTE. The actual resource allocation to a UE is made in terms of VRB pairs. There are two types of resource allocations, localized and distributed. In the localized resource allocation, a VRB pair is directly mapped to a PRB pair, hence two consecutive and localized VRBs are also placed as consecutive PRBs in the frequency domain. On the other hand, the distributed VRBs are not mapped to consecutive PRBs in the frequency domain; thereby providing frequency diversity for data channel transmitted using these distributed VRBs.
Downlink (DL) transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about to which terminals data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols (CRS), which are known to the receiver and used for coherent demodulation of e.g. the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
3.2 Carrier Aggregation
The LTE Rel-10 specifications (i.e. LTE Release 10 specifications; hereinafter Release is abbreviated Rel) have been standardized, supporting Component Carrier (CC) bandwidths up to 20 MHz (which is the maximal LTE Rel-8 carrier bandwidth). An LTE Rel-10 operation wider than 20 MHz is possible and appear as a number of LTE CCs to an LTE Rel-10 terminal, i.e. a UE supporting LTE Rel-10. The straightforward way to obtain bandwidths wider than 20 MHz is by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CCs, where the CC has, or at least the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 4.
The Rel-10 standard supports up to 5 aggregated CCs where each CC is limited in the RF specifications to have a one of six bandwidths namely 6, 15, 25, 50, 75 or 100 RB (corresponding to 1.4, 3 5 10 15 and 20 MHz respectively).
The number of aggregated CCs as well as the bandwidth of the individual CCs may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink (DL) and uplink (UL) is the same whereas an asymmetric configuration refers to the case that the number of CCs is different in DL and UL. It is important to note that the number of CCs configured in the network may be different from the number of CCs seen by a terminal: A terminal may for example support more downlink CCs than uplink CCs, even though the network offers the same number of uplink and downlink CCs.
CCs may also be referred to as cells or serving cells. More specifically, in an LTE network the cells aggregated by a terminal are denoted primary Serving Cell (PCell) and secondary Serving Cells (SCells). The term serving cell comprises both PCell and SCells. All UEs have one PCell. Which cell is a UEs PCell is terminal-specific and is considered “more important”, i.e. vital control signaling and other important signaling is typically handled via the PCell. Uplink control signaling is always sent on a UEs PCell. Thus, unless it can be multiplexed with other uplink data, uplink control signaling is sent on a UEs PCell. The component carrier configured as the PCell is the primary CC whereas all other component carriers are secondary serving cells.
During initial access a LTE Rel-10 terminal behaves similar to a LTE Rel-8 terminal, i.e. a terminal supporting LTE Rel-8. However, upon successful connection to the network a Rel-10 terminal may—depending on its own capabilities and the network—be configured with additional serving cells in the UL and DL. Configuration is based on Radio Resource Control (RRC). Due to the heavy signaling and rather slow speed of RRC signaling it is envisioned that a terminal may be configured with multiple serving cells even though not all of them are currently used.
3.3 SCell Activation and Deactivation
With the concept of SCells, i.e. a UE having more than one serving cell in carrier aggregation as discussed above, additional bandwidth resources could be configured/deconfigured dynamically. In this regard, it should be appreciated that the use of “/” is used to mean “and/or” throughout this disclosure. The configuration/deconfiguration of cells are signaled by the eNB and performed with RRC signaling which is heavy signaling and slow. Since RRC signaling is heavy and slow the concept of activation/deactivation was introduced for SCells. The eNB has the possibility to deactivate a UE's serving cells which the eNB decides which that UE should not use for the moment. Activation/deactivation is performed with MAC signaling which is faster. The activation/deactivation procedure is described in detail in section 5.13 of 3GPP TS 36.321, V10.5.0 (2012-03) 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification (Release 10). Each SCell is configured with a SCellIndex, which is an identifier or so called Cell Index which is unique among all serving cells configured for this UE. The PCell always have Cell Index 0 and and SCell can have an integer cell index of 1 to 7.
The Rel-10 Activation/Deactivation MAC control element (CE) is defined in section 6.1.3.8 of 3GPP TS 36.321, V10.5.0 (2012-03). The Activation/Deactivation MAC CE consists of a single octet containing seven C-fields and one R-field. Each C-field corresponds to a specific SCellIndex and indicates whether the specific SCell is activated or deactivated. The UE will ignore all C-fields associated with Cell indices not being configured. The Activation/Deactivation MAC CE always indicates the activation status of all configured SCells, meaning that if the eNB wants to activate one SCell it has to include all configured SCells, setting them to activated or deactivated even if their status has not changed.
The UE is generally required to monitor the control channel, PDCCH, and data channel, PDSCH, of every serving cell. In carrier aggregation a UE is configured with at least one SCell. The UE's configured SCell can be activated or deactivated by the serving network node. Therefore if a UE's SCell is activated it would imply that the UE has to monitor PDCCH and PDSCH for that serving cell. This implies a wider receiver bandwidth, higher sampling rates, etc. resulting in high power consumption compared to if that serving cell would have been deactivated.
3.4 Timing Alignment of Signals Received at eNode B
In order to preserve the orthogonality in UL, the UL transmissions from multiple UEs need to be received time aligned at the eNB. This means the transmit timing of the UEs, which are under the control of the same eNB, should be adjusted to ensure that their received signals arrived at the eNB receiver at the same time—more specifically well within the cyclic prefix (CP). This ensures that the eB receiver is able to use the same resources (i.e. same DFT or FFT resource) to receive and process the signals from multiple UEs.
Since UEs may be located at different distances from the eNB (see FIG. 5), the UEs will need to initiate their UL transmissions at different times. A UE far from the eNB generally needs to start transmission earlier than a UE close to the eNB. This can for example be handled by time advance of the UL transmissions, i.e. a UE starts its UL transmission before a nominal time given by a timing reference. This concept is illustrated in FIG. 6. In other words, a UE may start its UL transmission at a time, given by a timing advance value, before a nominal time given by a timing reference, or timing reference value.
The UL timing advance is maintained by the eNB through timing advance commands transmitted to a UE based on measurements on UL transmissions from that UE, i.e. the same UE. Through timing advance commands, the UE is ordered to start its UL transmissions earlier or later than current UL transmission timing. That is, the timing advance (TA) value can be increased or decreased, respectively.
There is a strict relation between DL transmissions and the corresponding UL transmission. Examples of this include the timing between a DL-SCH transmission on PDSCH to the HARQ ACK/NACK feedback transmitted in UL (either on PUCCH or PUSCH); and the timing between an UL grant transmission on PDCCH to the UL-SCH transmission on PUSCH.
By increasing the timing advance value for a UE, the UE processing time between the DL transmission and the corresponding UL transmission decreases. For this reason, an upper limit on the maximum timing advance has been defined by 3GPP in order to set a lower limit on the processing time available for a UE. For LTE, this value has been set to roughly 667 μs which corresponds to a cell range of roughly 100 km (note that the TA value compensates for the round trip delay).
In LTE Rel-10 there is only a single timing advance (TA) value per UE and all UL cells are assumed to have the same transmission timing. The timing reference point for the TA is the receive timing of the primary DL cell.
In LTE Rel-11, support for multiple TA values was introduced and one UE may have different TA values for different cells. One reason for the introduction of multiple TA values is that a UE should support UL transmission to multiple UL reception points. As in general a UE have different round trip delay to different physical nodes, the UE will, in general, need different TA values to these different physical nodes. A UE might also need different TA values for UL transmissions to cells in different bands. The UE's UL serving cells terminating at the same physical node or at the co-located physical nodes (e.g. co-located eNodeB) have same or similar TA values for said UE. Therefore the possibility to configure multiple TA groups (TAGs) has been specified in 3GPP standard and can be used when a UE has 2 or more UL serving cells. The criterion to group cells within a TA group (TAG) depends upon network implementation. For example, typically the UE's UL serving cells terminating at the same or co-located node are grouped by the network in the same TAG. TA grouping can thus be done for example depending on deployment where UL serving cells terminated at the same physical node will be grouped into the same TA group. In other words, the current assumption in 3GPP is that the serving cells of a UE which the eNB considers suit to use the same TA value will be grouped together in the so called TA group. TA grouping will be signalled by the network (NW) by RRC signalling. Or said differently, the information relating to TA grouping is signalled to the UE by the NW via RRC signalling. As is understood,
Serving cells in the same TA group will share TA value. Also, the DL of one serving cell in the TA group will be used as timing reference. Thus, the DL of one serving cell in the TA group will be used as timing reference for the UE for deriving its UL transmit timing of all cells belonging to the same TAG. To each TA value there is an associated timer, called TA timer. The UE assumes that all the UL serving cells in the same TA group to be time aligned when the associated TA timer is running. If the serving cells in the TAG are considered to be time aligned the UE is allowed to perform PUCCH, PUSCH and SRS transmissions on one or more of the serving cells. A TA timer is started or restarted upon reception of a TA command addressed to the associated TA group. TA commands are discussed further in section 3.6 hereof.
3.5 Random Access
In LTE, as in any communication system, a mobile terminal (e.g. UE) may need to contact the network (via the eNodeB) without having a dedicated resource in the Uplink (i.e. from UE to base station). To handle this, a random access procedure is available where a UE (that does not have a dedicated UL resource) may transmit a signal to the base station. The first message (MSG1 or preamble) of this procedure is typically transmitted on a special resource reserved for random access, a physical random access channel (PRACH). This channel can for instance be limited in time and/or frequency (as in LTE). See FIG. 7. The resources available for PRACH transmission is provided to the terminals as part of the broadcasted system information (or as part of dedicated RRC signaling in case of e.g. handover).
In LTE, the random access procedure can be used for a number of different reasons. Among these reasons are:                Initial access (for UEs in the LTE_IDLE or LTE_DETACHED states)        Incoming handover        Resynchronization of the UL        Scheduling request (for a UE that is not allocated any other resource for contacting the base station)        Positioning        
The contention-based random access procedure used in LTE is illustrated in FIG. 8. The UE starts the random access procedure by randomly selecting one of the preambles available for contention-based random access. The UE then transmits the selected random access preamble on the physical random access channel (PRACH) to eNB in RAN.
The RAN acknowledges any preamble it detects by transmitting a random access response (MSG2) including an initial grant to be used on the uplink shared channel, a temporary C-RNTI, and a time alignment (TA) update based on the timing offset of the preamble measured by the eNodeB on the PRACH. The MSG2 is transmitted in the DL to the UE and its corresponding PDCCH message CRC is scrambled with the RA-RNTI.
When receiving the random access response (MSG2) the UE uses the grant to transmit a message (MSG3) that is used, in part, to trigger the establishment of radio resource control and, in part, to uniquely identify the UE on the common channels of the cell. The timing advance command provided in the random access response is applied in the UL transmission in MSG3. The eNB can change the resources blocks that are assigned for a MSG3 transmission by sending an UL grant that's CRC is scrambled with the TC-RNTI.
The MSG4 which is then used for contention resolution has its PDCCH CRC scrambled with the C-RNTI if the UE previously has a C-RNTI assigned. If the UE does not have a C-RNTI previously assigned has its PDCCH CRC is scrambled with the TC-RNTI. That is to say, if the UE is not previously assigned a C-RNTI then the CRC of the PDCCH contained in the MSG4 is scrambled with the TC-RNTI.
The procedure ends with RAN solving any preamble contention that may have occurred for the case that multiple UEs transmitted the same preamble at the same time. This can occur since each UE randomly selects when to transmit and which preamble to use. If multiple UEs select the same preamble for the transmission on RACH, there will be contention between these UEs that needs to be resolved through the contention resolution message (MSG4). The case when contention occurs is illustrated in FIG. 9, where two UEs transmit the same preamble, p5, at the same time. A third UE also transmits at the same RACH, but since it transmits with a different preamble, p1, there is no contention between this UE and the other two UEs.
The UE can also perform non-contention based random access. A non-contention based random access or contention free random access can e.g. be initiated by the eNB to get the UE to achieve synchronisation in UL. The eNB initiates a non-contention based random access either by sending a PDCCH order or indicating it in an RRC message. The later of the two is used in case of handover (HO).
The eNB can also order the UE through a PDCCH message to perform a contention based random access; the procedure for this is illustrated in FIG. 9. The procedure for the UE to perform contention free random access is illustrated in FIG. 10. Similar to the contention based random access the MSG2 is transmitted in the DL to the UE and its corresponding PDCCH message CRC is scrambled with the RA-RNTI. The UE considers the contention resolution successfully completed after it has received MSG2 successfully.
For the contention free random access as for the contention based random access the MSG2 contain a timing alignment value. This enables the eNB to set the initial/updated timing according to the UEs transmitted preamble.
In LTE Rel-10, the random access procedure is limited to the primary cell only. This implies that the UE can only send a preamble on the primary cell. Further MSG2 and MSG3 is only received and transmitted on the primary cell. MSG4 can however, in Rel-10, be transmitted on any DL cell.
In LTE Rel-11, the current assumption (RAN2#75, August 2011) is that the random access procedure will be supported also on secondary cells, at least for the UEs supporting Rel-11 carrier aggregation (CA). So far only network initiated random access on SCells is assumed.
3.6 Initial TAC and Subsequent TAC
TA values are used by the UE to offset the UL transmission timing relative to a reference. The current assumption in 3GPP is that the DL reception timing of a serving cell is used as timing reference and the UL transmission timing will be offset relative to the DL reception timing of that, so called, timing reference cell. At preamble transmission the UE uses a TA value of zero and the preamble will therefore be transmitted at the time of DL reception of the timing reference cell. When the eNB receives the preamble it measures the time misalignment of wanted UL reception timing on the cell on which the preamble was transmitted and the actual UL timing of the preamble. Based on this measured misalignment the eNB creates an initial TA command (TAC) which is sent to the UE in the random access response message (MSG2). When the UE receives this TA command it will apply the indicated TA value to the TA group which the cell which performed the preamble transmission. The TA value tells the UE how much to advance the UL transmission in subsequent UL transmissions on the cells belonging to that TA group.
Because a UE can move and the round trip time to the UL reception points can change, and the TA values might then become inaccurate. Therefore, when receiving UL transmissions from a UE on a cell the eNB measures the time misalignment of the UL signals from that UE on that cell. If measured time misalignment of the UL signals from that UE on a cell is, by the eNB, judged to be too large, the eNB can create a TA command message containing a delta update to the TA value used by that UE. The UE will, when receiving such a TA command, increase or decrease the TA value according to the delta update.
The initial TA command is an 11 bit long value and is sent in the random access response message. An initial TA command tells the UE how much the addressed TA value should be advanced. The addressed TA value is the TA value which is associated with the TA group to which the cell where the preamble was sent, or put in other words. If a UE perform random access on a cell belonging to a TA group x then the TA value associated with TA group x is the addressed TA value. Subsequent TA commands are 6 bit long values and are sent in TA command MAC Control Elements (CEs) which aside from the TA command itself also contains a TA group identity and the TA value associated with the identified TA group is the addressed TA value. A TA command tells the UE how much the TA value should be advanced.
It has recently been agreed in 3GPP that for the serving cells in the same TA group as the PCell the DL reception timing of the PCell should be the timing reference. So, in a TAG containing PCell (also known as PCell TAG (pTAG)), the UE uses PCell as the reference cell for deriving the UE transmit timing for all serving cells in the pTAG. When the UE is configured with a TAG containing only SCells (also known as SCell TAG (sTAG)), the UE may use any activated SCell from the sTAG for deriving the UE transmit timing for all SCells in the sTAG. For serving cells in a TA group not containing the PCell the DL reception timing of a serving cell selected by the UE should be used as timing reference.
When receiving a TA command, initial or subsequent, the UE will apply the TA command and start the associated TA timer. The UE will consider the serving cells belonging to a TA group as UL in-synch, i.e. UL time aligned, as long as the associated TA timer is running. When the UE is considering a cell UL time aligned normal UL transmissions are allowed while when a cell is not considering UL time alignment only PRACH transmissions are allowed.
3.7 Autonomous Uplink Timing Adjustment
In addition to the TA based adjustment of the UL transmit timing, in the existing solutions there is also pre-defined requirement on the UE to autonomously adjust its UL timing in response to the drift in the eNB transmit timing. More specifically the UE is required to follow the change in the frame transmit timing of the serving cell and correspondingly adjust its transmit timing for each transmission. The UE typically uses some sort of reference signals to track the downlink timing of the serving cell e.g. common reference signal, synchronization signals, etc.
The serving cell timing may change due to different reasons e.g. due to variation in radio conditions, imperfection in clocks, maintenance activities, deliberate attempt by the network to change timing etc. In addition it is also required that the UE changes its timing (increase or decrease) with certain rate. This is to make sure that the UE does not change the timing too fast. This requirement stems from the fact that if the UE changes its timing in the order of several μs (e.g. 3-4 μs) from subframe to subframe, the base station receiver may not be able to cope with the received signals. This will result in degradation of demodulation of signals transmitted by the UE e.g. result in uplink throughput loss.
This autonomous UL timing adjustment is (in LTE) realized by the fact that each TA value (NTA) has a reference TA value (NTA_Ref). The UE is (in LTE) required to follow the frame timing change of the serving cell or of the timing reference cell if there are more than one serving cell in a TAG. This is also called the autonomous UL timing adjustment since it is autonomously performed by the UE based on pre-defined rules and requirements. The uplink frame transmission takes place (NTA+NTA offset)×Ts before the reception of the first detected path (in time) of the corresponding downlink frame from the reference cell. Where Ts is a basic time unit defined as: Ts=1/(15000×2048) seconds. NTA is the currently used TA value. NTA_Ref is the TA value which has been signaled by the network and is used to determine the reference timing of UL transmission. This reference timing is (NTA_Ref+NTA_offset) before the downlink reception timing. NTA_offset is a fixed value. When multiple TAGs are used the UE is required to autonomous adjusts its UL timing adjustment for UL transmissions for cells in each TA group. Therefore each TA in a TAG is associated with a reference TA value (NTA_Ref). The goal of the autonomous uplink timing adjustment is to reduce the difference between the reference timing and the actual uplink timing below a certain threshold. If the downlink reception timing changes by a time t the reference timing will be changed by a time t in the same direction which can result in an increase in the difference between the reference timing and the actual UL timing. This is illustrated, for example, in FIG. 11 and FIG. 12. At time t1 shown in FIG. 11 there is a small difference between the reference timing and the actual UL timing, i.e. Te is small. FIG. 12 illustrates a possible later state at time t3 where the DL timing has been delayed, which could for example happen due to that the DL transmission timing has been delayed. If Te is larger than a threshold the UE is required to adjust the UL transmission timing. This is done by adjusting NTA and is illustrated in FIG. 16.
Whenever a TAC MAC CE is received the UE will update, i.e. advance or delay, the currently used UL transmission timing according to the value in the received TAC and the reference timing is changed to be the updated UL transmission timing. Or in other words; whenever a UE is applying the value of a TAC MAC CE to the addressed cell and/or TAG the UE will update the UL transmission timing of that cell and/or TAG, update NTA to be DL timing minus the new, updated, Actual UL timing and NTA_Ref so that NTA_Ref+NTA_offset=NTA.
4.0 Problems, or Disadvantages, with Existing Technology
With the current LTE specification the UE will only perform autonomous UL transmission timing adjustments when performing UL transmission. If the UE is not performing any UL transmissions for a period of time during which the DL timing has drifted there is a risk that Te (transmission timing error) has increased above the threshold for triggering of autonomous UL transmission timing adjustment. The UE adjust UL transmission timing when it performs UL transmission. Due to inactivity or lack of UL grant the UE may not transmit in every subframe or transmission occasion. Therefore, the next UL transmission timing adjustment will be performed by the UE at the next UL transmission.
When a TA group's TA timer has expired, the UE is prohibited to perform any UL transmissions and it would therefore not perform any UL transmission timing adjustment for that TA group. That is to say that when a TA group's TA timer has expired, the UE is generally prohibited from performing any UL transmissions on the cells in that TAG and it would therefore not perform any UL transmission timing adjustment for the TA group in question.
One possible procedure the eNB can use to manage a UE's UL transmissions on a serving cell is by utilizing the TA timer associated with that serving cell. If the eNB wants to stop a UE's UL transmission on a serving cell it could stop sending TAC MAC CEs addressed to that serving cell's TA group and hence the TA timer would expire (given that the timer has a finite value) and thereby the UE would not be allowed to perform UL transmissions on the serving cells in that TA group. If at a later stage the eNB wants that UE to resume UL transmissions on a serving cell belonging to a TA group with an expired TA timer it could send a TAC MAC CE addressed to that serving cell's TA group which would update the associated TA value and start the associated TA timer. In such a scenario it is likely that the eNB has, by some means, interpreted the UE as non-moving, or slowly moving, so that the TA values maintained by the UE would still be valid even though the associated TA timer has expired. A problem arises if the DL transmission timing has drifted during the period of time when the TA timer has not been running and the UE is therefore not allowed to perform UL transmission and therefore not performing autonomous UL transmission timing adjustments to compensate for the drift in DL reception timing. The actual UL timing would in such a scenario be faulty and if the UE receives a TAC MAC CE the UE will update this, faulty, actual UL timing according to the TAC MAC CE and restart the associated TA timer which would resume UL transmissions on that serving cell with a faulty TA value which may lead to substantial performance degradation at the base station receiver in case there is a large error in UL transmission timing, e.g. in the order of few μs such as 2-4 μs. The problem is further accentuated in case of for example when there is sudden change in propagation conditions, drift due to imperfection in clocks, maintenance work at the base station etc. Hence the aggregated time misalignment may momentarily become significantly large. Therefore there is a need to define a mechanism to ensure that the UE does not use faulty UL transmission timing in the mentioned scenario.